This invention relates to a method for dividing a density range of component dyes of a color hard copy, particularly in the case where it is necessary to obtain the color hard copy from quantized color image signals.
Generally, when it is required to prepare or obtain a color hard copy from quantized color image signals, for example, where each signal consists of three bits (eight levels) for each of the component colors of blue B, green G and red R, the image quality of the obtained color hard copy is largely and significantly influenced by the manner in which the density range of the component dyes of yellow Y, magenta M and cyan C is divided according to the color image signals.
In a conventional method, a color hard copy is prepared by dividing density range (Dmax-Dmin) of the component dyes of Y, M and C by a number (N-1), with N being the "level number", to thereby equally divide the density of the component dye according to the levels of color image signals.
In the conventional method described above, however, it is difficult to obtain the color hard copy with the appropriate lightness reproduction especially in the high light range, where the lightness gap tends to be too great. There is also no guarantee in obtaining the exact gray reproduction. Accordingly, a method is disclosed in which the density range of the component dyes is equally divided while maintaining the density ratio of the dyes Y, M and C constituting the gray color as shown in FIG. 4. However, in the lightness reproduction as represented by the curve A shown in FIG. 1, of gray in the color hard copy, the variation of the lightness decreases as the level of the color image signal becomes large. This tendency also applies to the other colors as well as gray.
FIG. 2 shows a graph representing the relationship between a color difference number j and a color difference .DELTA.E.sub.j.sup.i on a correspondingly obtained color hard copy in the case where the density range of the component dye is equally divided. In the graph, the level values j and j+1 of the color image signals correspond respectively to the colors C.sub.j.sup.i and C.sub.j.sup.i +1 (i shows a color) on the color hard copy, and the difference therebetween is represented by a color difference .DELTA.E.sub.j.sup.i (j is a color difference number). The color difference number is from members 1 to 7 in accordance with the level values 1 to 8, and the colors i, i.e., the seven colors of B (blue), G (green), R (red), Y (yellow), M (magenta), C (cyan) and K (black).
As is apparent from FIG. 2, the color difference .DELTA.E.sub.j.sup.i is small in a dark region in which the color difference number j is large, whereas the color difference .DELTA.E.sub.j.sup.i is large in a light region in which the color difference number is small.
For the reasion described above, in the color reproduction of a color hard copy based on the color image signal where the density of the component dye is equally divided, a large color change (i.e. very large color difference) is observed in the light color region and a small color change (i.e. very small color difference) is observed in the dark color region, thus attaining a poor color continuity.
FIG. 3 shows a graph representing the relationship between the color difference .DELTA.E.sub.j.sup.i and the frequency of the occurrence of that color difference. The graph shows that the frequencies are more or less similar to each other for all the color differences .DELTA.E.sub.j.sup.i. This shows that the portions having the large color difference and those having small color difference are distributed equally throughout the entire color image. Therefore, according to the conventional method, a color hard copy having a visually excellent reproduction of the color image is not obtained.
The nature, principle and utility of the invention will become more apparent from the following detailed description in conjunction with the accompanying drawings.